81
hearing sensitivity in C. auriga does not match the lower frequency band of the only
sound (head-bob jaw-protrusion) recorded to date for this species in the laboratory. It
is possible that higher frequency sounds are produced in the fi eld by C. auriga as
observed in C. ornatissimus (Tricas and Boyle 2014 ). In addition, a biological function, if any, for the extended frequency range of hearing (up to 2 kHz) for some
Chaetodon sound types remains to be demonstrated. In addition, the 200–300 Hz
band of best sensitivity in late larval and juvenile spotfi n butterfl yfi sh, C. ocellatus
(Webb et al. 2012 ), is also well below the upper 1–3 kHz range reported for species
highly sensitive to sound pressure (reviewed by Ladich and Fay 2013 ), thus the
potential use of sound pressure stimuli by larval butterfl yfi sh may be more limited in
bandwidth.
Detailed morphological studies of the ear in Forcipiger fl avissimus and in
several Chaetodon species with different LC variants confi rm that there is no
intimate association of the swim bladder horns with the otic capsule, or notable
modifi cation of the ear (Webb et al. 2010 ). However, the swim bladder horns of
both C. auriga and C. multicinctus are long and have similar lengths when corrected for body size (Woods 2006 ). Further, the swim bladder horns of C. multicinctus are closer to the ear (1 mm) than those in C. auriga (2 mm). The proximity
of the horns to the ear and the shape of the swim bladder horns in species with
Indirect LC variants may explain the stronger effect of the horns on auditory
sensitivity in C. multicinctus (and likely other species with Indirect LC variants)
than in C. auriga (and other species with Direct LC variants, Woods 2006 ).
Subsequent evacuation of gas from the swim bladder further reduced hearing
sensitivity in C. multicinctus , but not in C. auriga (Fig. 10 ; Tricas and Boyle
2015b ). Of particular interest is that the highest frequency sensitivity found
among butterfl yfi shes (2 kHz) was observed in C. ornatissimus , a species with
short swim bladder horns (LC variant Ind2) that approach the ear to within a
distance of about 1 mm (Woods 2006 ). Further modeling and experiments are
needed to demonstrate the frequency-dependent displacement amplitudes of the
swim bladder horns in three axes that are caused by sound pressure stimuli, and
to determine their physical contribution to the extended hearing sensitivity and
frequency range of Chaetodon .
Fig. 10 (continued) Subsequent defl ation of the swim bladder demonstrated further threshold
increases most notable at 600 Hz. ( d ) C. auriga has long swim bladder horns with a direct connection to the LC. Baseline thresholds increased by about 10 dB at 200–600 Hz after gas was evacuated from the swim bladder horns and swim bladder. AEP threshold data are provided in relation
to sound pressure ( left column ) and particle acceleration ( right column ). Data are means and SE
among individuals. Numbers at circles indicate sample size at each test frequency, or fraction of
test subjects for which an AEP response was recorded. From Tricas and Boyle ( 2015b )
Acoustic Communication in Butterfl yfi shes…
hearing sensitivity in C. auriga does not match the lower frequency band of the only
sound (head-bob jaw-protrusion) recorded to date for this species in the laboratory. It
is possible that higher frequency sounds are produced in the fi eld by C. auriga as
observed in C. ornatissimus (Tricas and Boyle 2014 ). In addition, a biological function, if any, for the extended frequency range of hearing (up to 2 kHz) for some
Chaetodon sound types remains to be demonstrated. In addition, the 200–300 Hz
band of best sensitivity in late larval and juvenile spotfi n butterfl yfi sh, C. ocellatus
(Webb et al. 2012 ), is also well below the upper 1–3 kHz range reported for species
highly sensitive to sound pressure (reviewed by Ladich and Fay 2013 ), thus the
potential use of sound pressure stimuli by larval butterfl yfi sh may be more limited in
bandwidth.
Detailed morphological studies of the ear in Forcipiger fl avissimus and in
several Chaetodon species with different LC variants confi rm that there is no
intimate association of the swim bladder horns with the otic capsule, or notable
modifi cation of the ear (Webb et al. 2010 ). However, the swim bladder horns of
both C. auriga and C. multicinctus are long and have similar lengths when corrected for body size (Woods 2006 ). Further, the swim bladder horns of C. multicinctus are closer to the ear (1 mm) than those in C. auriga (2 mm). The proximity
of the horns to the ear and the shape of the swim bladder horns in species with
Indirect LC variants may explain the stronger effect of the horns on auditory
sensitivity in C. multicinctus (and likely other species with Indirect LC variants)
than in C. auriga (and other species with Direct LC variants, Woods 2006 ).
Subsequent evacuation of gas from the swim bladder further reduced hearing
sensitivity in C. multicinctus , but not in C. auriga (Fig. 10 ; Tricas and Boyle
2015b ). Of particular interest is that the highest frequency sensitivity found
among butterfl yfi shes (2 kHz) was observed in C. ornatissimus , a species with
short swim bladder horns (LC variant Ind2) that approach the ear to within a
distance of about 1 mm (Woods 2006 ). Further modeling and experiments are
needed to demonstrate the frequency-dependent displacement amplitudes of the
swim bladder horns in three axes that are caused by sound pressure stimuli, and
to determine their physical contribution to the extended hearing sensitivity and
frequency range of Chaetodon .
Fig. 10 (continued) Subsequent defl ation of the swim bladder demonstrated further threshold
increases most notable at 600 Hz. ( d ) C. auriga has long swim bladder horns with a direct connection to the LC. Baseline thresholds increased by about 10 dB at 200–600 Hz after gas was evacuated from the swim bladder horns and swim bladder. AEP threshold data are provided in relation
to sound pressure ( left column ) and particle acceleration ( right column ). Data are means and SE
among individuals. Numbers at circles indicate sample size at each test frequency, or fraction of
test subjects for which an AEP response was recorded. From Tricas and Boyle ( 2015b )
Acoustic Communication in Butterfl yfi shes…